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29 Sep 2011, 15:38

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Props to Arzad.I got C but with a different, yet slightly longer approach, which I constantly use for proportion problems such as this one. Usually, this approach is very efficient when used with slightly difficult questions, but this problem is quite hard so I had to slightly tweak it. I will show you how I usually use the approach and later show you how to use it with this specific problem.

Example. 1 Liter of Solution A contains 45% alcohol, while 1 Liter of Solution B contains 20% alcohol. In what ratio must the two solutions be used to get a solution with 30% alcohol

Now using this same approach, we tackle Gopu106’s question. It is important to first think of X in the mixture as the alcohol in the problem above; hence, a mixture of X and Y in the ratio of 3:2 translates to X is 3/5 of the solution. Applying this concept to all three equations, we write:1. 3/5*[A/(A+B)]+3/7*[B/(A+B)] = 5/92. Now here is the tweak that must be made to continue with this approach. You must find the common denominator for all three numbers and organize the fractions accordingly. By finding the common denominator of 5,7,9 (or 315) we re-write the equations as follows3. 189/315*[A/(A+B)]+135/315*[B/(A+B)] = 175/3154. Multiply 315 to both sides to arrive at 189A/(A+B) + 135B/(A+B) = 1755. Multiply (A+B) to both sides to arrive at 189A + 135B = 175A + 175B6. Distribute to arrive at 14A = 40B7. Thus the ratio is A/B = 40/14 = 20/7 or answer C

Finding the common denominator and adjusting the numerator is time consuming, but knowing some number property rules would speed the process. For example, if you know that your common denominator is (5)(7)(9), and you want to apply this to 3/5, then you just multiply 3*(7)(9) and omit the (5) because that is already present in the denominator and arrive at 189/315.Again, this process is much longer than that of Arzad’s, but it is always good to know how to solve a problem multiple ways.

Re: Two mixtures of X and Y have X and Y in the ratio 3:2 and 3 [#permalink]

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16 Dec 2013, 06:36

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Shortest Method(based on Manhanttan approach):

According to the shortcut, " Suppose that A and B are averaged together. If they are in a ratio of a: b, then you can multiply the differential of A by a, and it will cancel out with the differential of B times b."

For ex:suppose there is a group of men and women in a ratio of 2:3. If the men have an average age of 50, and the average age of the group is 56, you can easily figure out the average age of the women in the group. Men have a - 6 differential, and there are 2 of them for every 3 women. If the average age of women is w , then:

2 x (-6) + 3 x (w) = 0 -12 + 3w = 0w = 4Women have a +4 differential. The average age of the women in the group is 56 + 4 = 60 years old.

Applying the idea in our problem,(3/5-5/9)X - (5/9-3/7)Y = 0[Here negative sign is applied for 'Y' as its value is lower than the average(3/7<5/9)], which gives us:X/Y=20/7

This technique works on almost all of the problems involving Mixtures.

Piyush K-----------------------Our greatest weakness lies in giving up. The most certain way to succeed is to try just one more time. ― Thomas A. EdisonDon't forget to press--> KudosMy Articles: 1. WOULD: when to use?| 2. All GMATPrep RCs (New)Tip: Before exam a week earlier don't forget to exhaust all gmatprep problems specially for "sentence correction".

Re: Two mixtures of X and Y have X and Y in the ratio 3:2 and 3 [#permalink]

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13 Aug 2015, 07:06

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Re: Two mixtures of X and Y have X and Y in the ratio 3:2 and 3 [#permalink]

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13 Aug 2015, 23:27

g106 wrote:

Two mixtures of X and Y have X and Y in the ratio 3:2 and 3:4. In what proportion should these two mixtures be mixed to get a new mixture in which the ration of X to Y is 5:4?

A. 6:1B. 5:4C. 20:7D. 10:9E. 14:11

x in the first mixture is 3/5 or 189/315x in the second is 3/7 or 135/315x in the resultant mix is 5/9 or 175/315(Go for a convenient number which is 315)The required ratio is (175-135):(189-175)40:14 or 20:7